Proportional editing excluding certain faces
In this case one can either edit the left or right side of the objects location. This is because X Mirror is active.



Disable it by pressing T to open the tools panel. Go to the options tab and uncheck X-Mirror.

For further reference, also make sure you're the "Apply shape in edit mode" option of shapekeys as that could change edit mode behaviour as well and you have chosen the correct proportional editing mode.